ALMOST 5000 NSW teachers are risking suspension because they've failed to say if they're fully vaccinated against COVID–19, days before a vaccine mandate kicks in.

About 4900 teachers haven't told the Education Department if they're fully vaccinated, the department's chief people officer Yvette Cachia told a budget estimates hearing in NSW Parliament on Tuesday.

Teachers who aren't fully vaccinated will be suspended from Monday, when a public health order mandating vaccination for the state's teachers kicks in.

Ms Cachia said it was not surprising many were leaving it to the last minute, given the international experience, and she expected many teachers to attest to their vaccination status in the coming days.

Almost 74,000 teachers are registered as fully vaccinated.

One Nation MP Mark Latham questioned why rapid antigen tests couldn't be used for unvaccinated teachers, instead of suspending them.

"Rapid antigen testing certainly plays a role ... but it's not in place of vaccination," Education Minister Sarah Mitchell said on Tuesday.

Vaccinating teachers was the best way to keep students and teachers safe, especially when children under 12 weren't eligible to get the jab, she said.

Department secretary Georgia Harrisson said rapid antigen tests would not protect a teacher from being infected.

"Even if a staff member took a rapid antigen test and was not positive for COVID, they would be in a classroom where there could be a child that put them at risk," she said.

Rapid antigen testing for schools is being trialled in the border town of Albury, which has a COVID–19 outbreak.

It's being used as surveillance in the community to make sure the outbreak is contained, and to ensure there isn't transmission in schools, Ms Harrisson says.

The quicker, less reliable form of testing could also be used to limit the amount of time those exposed to the virus need to spend self–isolating.

The department is contracting about 30 short–term workers to act as investigators to look into teachers' non–compliance with vaccine requirements once the mandate applies.

Ms Cachia said the surge workforce was to help complete investigations in a timely fashion to get teachers back into the classroom if the issue could be worked out.

"We want to be able to give certainty to our school principals on staffing," she said.

"What we are trying to do is to ensure that before any decisions around employment are taken ... that we have fair and due process for our staff," Ms Harrisson said.

"That's why we are increasing our capacity to do so. We want to make sure that ... we are hearing every individual case appropriately."